The present invention pertains to fluoroescence analyzers and more particularly fluorescence analyzers used in determining sulfur dioxide content in a sample stream of gas.
In sulfur dioxide analyzers, the sample input stream containing sulfur dioxide to be analyzed is presented to a reaction chamber in which it is illuminated by an ultraviolet light source. Sulfur dioxide molecules present absorb the incident radiation, increase in energy content momentarily and then release the absorbed energy at a longer wavelength than the incident radiation (fluorescence). The fluorescence radiation is detected at right angles to the incident radiation by a photomultipler tube and electrically amplified to be displayed as the signal proportional to the concentration of sulfur dioxide present in the input gas sample. Further details of fluorescent methodology may be found disclosed in U.S. Pat. No. 3,795,812 to Hideo Okabi.
Other compounds exist in sample streams containing sulfur dioxide which also fluoresce in a similar fashion to the fluorescence exhibited by sulfur dioxide. Polynuclear aeromatic hydrocarbons (such as naphthalene, anthracene, penanthracene, etc.) were generally considered the principal class of compounds which exhibit this behavior, for which a method and apparatus to remove their interferent effects is disclosed in copending U.S. Patent application Ser. No. 12,174 to John N. Harman III, assigned to the same assignee as this application. However, the interferent effects of nitric oxide are now realized to produce interferent fluorescence and are not removed by existing prior art methods or apparatus. Nitric oxide fluoresces in a manner similar and at a similar wavelength to sulfur dioxide. While most applications of sulfur dioxide fluorescent analyzers have only negligible effects from nitric oxide, applications of sulfur dioxide fluorescent analyzers, such as measuring sulfur dioxide in automobile exhaust emissions, are plagued with nitric dioxide fluorescence to the extent that the indication of sulfur dioxide may be double its actual concentration.
From known prior art, the only solution to this problem is to perform a fluorescent analysis to determine the sulfur dioxide content of the sample and to perform a nitric dioxide analysis by a method other than fluorescence analysis. The nitric oxide content may then be subtracted from the indicated sulfur dioxide content to achieve a true sulfur dioxide content.